Porous ceramic materials have been made previously by adding organic materials to the ceramic during fabrication, and then burning out the organic materials to leave holes or voids therein. For example, in brick manufacturing, sawdust or wood powder (i.e., “fugitive material”) has been added to make the bricks lighter. On firing, the sawdust burns out, leaving a void where the wood once resided. This methodology has a number of drawbacks. In particular, it requires a significant amount of additional energy input (e.g., heating of the ceramic to combust the sawdust) and time. If the bricks are fired too fast, burning sawdust can cause the bricks to fracture. In addition, the burning process can leave a significant amount of residue to remove from the resulting porous ceramic material depending on the time and temperature of heating. Finally, the fugitive material adversely impacts profitability for the manufacturer by requiring space in the plant to store the material before use. This involves both an expense for storage and a decrease in the production capacity of the plant since some of the area is used up for storage purposes.
Porous polymeric materials can be made by a process wherein hollow glass spheres are combined with the polymer forming material. The density of the polymer composite can be varied by varying the density of the glass spheres and the volume fraction of spheres added. However, similar to prior porous ceramic structure manufacturing techniques, techniques for forming porous polymeric materials suffer from the fact that a very large volume of usable plant space must be reserved to store the large volumes of the glass spheres. This usage of space for storage of the glass spheres carries a very high cost both in overhead costs and in lost production capacity.
U.S. Pat. No. 5,071,747 to Hough et al. describes porous polymeric material, which in one embodiment, includes yeast within the pores of the material. The Hough invention contemplates formation of an emulsion from monomers and pre-polymers, followed by polymerizing the monomers and pre-polymers to yield a porous polymeric material, and finally, incorporation of the yeast within the pores. The yeast does not function to create the pores in the Hough device. Rather, the yeast provides biological activity in the device that is ultimately produced.
U.S. Pat. No. 4,603,111 to Keller describes a process for making a polyacrylamide bead, which in one embodiment incorporates yeast. The process involves combining the yeast with the acrylamide monomers, followed by polymerization. The resulting product is a bead with yeast immobilized thereon and therein. It was determined that the immobilized yeast retained the same activity of non-immobilized yeast. Thus, the beads could then be ideally used in later processes. It is noted that the yeast in Keller do not function in any capacity to form pores in the polyacrylamide bead.
U.S. Pat. No. 5,705,118 to Hayes describes a process for forming a ceramic material which includes combining an organic material such as gluten with a ceramic, followed by firing the ceramic material to form a green body. The gluten functions as a binder, and is ultimately eliminated by heat treatment in the manner discussed above in conjunction with prior technologies. Hayes indicates that minor amounts of yeast or enzymes, as well as many other constituents, may be included with the gluten, and that a “risen loaf” from the ceramic/gluten/yeast or enzyme mixture is ultimately fired.
U.S. Patent Publication 2003/0171822 to Lo describes a process for creating a porous synthetic bone graft wherein ceramic powder, binder and a pore-forming agent are combined in an inert liquid. The pore forming agent is then allowed to create the pores, and the porous structure is then fixed by a heat treatment. A high temperature heating is then used to eliminate the binder and the pore forming agent, and to fuse the structure together. Lo suggests that the pore forming agents could be yeast cells, alkali metal salts, and inorganic salts of acids derived from carbon and phosphorous. Lo indicates that a carbohydrate powder, and, in the case of yeast being used as the pore forming agent, sugar, are added to the slurry. Lo does not contemplate combining metal materials with the ceramic. Rather, in Lo, it is important to have pores in order to allow osteoblasts to attach in order to promote mineralization. Further, the Lo process methodology is not applicable to polymer materials as it requires the use of a binder, and a post pore forming, ceramic fusing heat step. Likewise, the Lo process suffers from the requirement of using carbohydrate powders and binders, which may result in porous structure where the pores are less uniform in size and/or are larger in size than is desired for certain industrial applications as opposed to applications in the human body.
It would be an advance in the art to provide a method for making porous materials that does not require large volumes of fugitive material. It would also be an advance to provide methods for making porous materials that can be used with metals, semiconductors, and polymers.